Summary of Work: The long-term goal of this project is to understand the fidelity of DNA synthesis by multiprotein DNA replication and repair complexes. This year, progress was made in three areas. We determined the fidelity of DNA synthesis by yeast DNA polymerase epsilon, one of the two major polymerases responsible for replicating the eukaryotic genome. To investigate the role of DNA polymerase epsilon in cells, we determined the influence of a point mutation in the active site of DNA polymerase epsilon on mutation rates in yeast strains with a variety of defects in other replication proteins. We also investigated the roles of yeast DNA polymerases delta and epsilon in polymerization and error correction during translesion synthesis past a thymine dimer, a photoproduct resulting from exposure to sunlight and responsible for the mutagenesis underlying susceptibility to skin cancer. Background: DNA polymerases do not work alone in cells, but function within interconnected DNA processing pathways that require many other proteins to replicate DNA and to repair various types of environmental DNA damage. Some of these proteins determine which substrates a DNA polymerase can copy and/or influence the efficiency and fidelity with which DNA synthesis occurs. Building on our structure-function studies of purified DNA polymerases, the goal of this project is to understand the fidelity of DNA synthesis by multi-protein DNA replication and DNA repair complexes, and to understand how perturbing these complexes may result in genome instability associated with environmental disease. A critical component of this effort is to connect in vi